The present disclosure is directed to an apparatus and a method for measuring the complex dielectric constant of a core sample When drilling a well, a core sample is periodically taken, and especially is obtained during drilling at or near formations of interest. The core sample is retrieved to the surface and various tests are made on the sample. The test results suggest certain drilling procedures including whether or not further drilling should be conducted, and also suggest implementation of various and sundry completion techniques. Since such data is useful, measuring various parameters of the core sample becomes quite important in the drilling procedure.
One of the important procedures which is typically carried out with geological formations is to measure the dielectric properties of the formation. Also, this is done with core samples. The dielectric constant however is not a simple variable; rather, it is a complex variable having resistive and reactive components. The typical form of the dielectric constant is A+jB where A and B are vector components at a 90.degree. angle, and further where B is the reactive component which is also typically related to bulk resistivity. In many instances, a well, partially or wholly drilled, yields one or more core samples which have to be tested in a laboratory facility, and the present procedure sets forth a method and apparatus for testing core samples and, particularly, cylindrical core samples. Both A and B are typically variable with frequency.
The core sample testing procedure begins in the field where a cylindrical core sample is first obtained. It is delivered to the laboratory or even to a field location where the present apparatus is located. The equipment utilizes a cylindrical wave guide which has three coupled but separated coils therein. The first or center coil serves as the transmitter coil. It includes one or more turns which are connected and deployed for propagation of a signal in both directions along the wave guide. First and second duplicate receiving coils are arranged parallel to the transmitting coil and are spaced from the transmitting coil by equal distances. Indeed, these distances are preferably held quite close so that the two receiving coils receive the signal propagated from the transmitting coil over equal distances. The received signals are picked up at the two receiving coils. They form output signals. The two output signals are delivered to an adder circuit. The receiving coils and the adder circuit are arranged so that two received signals substantially cancel, and the difference signal is then output from the adder to a network analyzer or a vector voltmeter. This measures the complex impedance and enables representation of the sample load in the form of A+jB. A signal generator connected with an amplifier is incorporated in the network analyzer to control timing so that the system operates in a synchronized fashion. Output signals are provided to the network analyzer from the transmitter coil and the adder. By proper calibration, the displayed signals at the analyzer can be evaluated and the A+jB complex dielectric constant can then be determined. This constant is in part dependent on frequency, and accordingly, the signal generator can be adjusted in frequency so that a selected frequency band can be tested. When this is done, different values are obtained to the extent the sample is frequency sensitive.
One of the significant advantages of the present system is the incorporation of the sample in a "circuit" without electrode contact. This is, the sample is placed in a circuit but this is accomplished without attempting electrode connection. Electrode contacts are difficult to connect to core samples.
Representative contact procedures are set out in U.S. Pat. Nos. 3,774,237 and also 3,895,289. Such core sample shaping and contact attachment are inconsistent and require much more handling.
Somewhat different approaches are also described at the article by Fuller & Ward reported at IEEE Transactions on Geoscience Electronics, Vol. GE 8, No. 1, January 1970 or the 1953 Geophysics article of Ward. A more recent approach is found in the Rau & Wharton article reported at Journal of Petroleum Technology, November 1982, pages 2689-2700. These however do not involve the disclosed process of placing the core sample in a guide and comparing the sample signal with a signal from a reference, and outputting a value of A+jB form.